1. Field of the Invention
The present invention relates to an optical coupler for highly efficiently guiding light from a light source such as a light emitting diode, a semiconductor laser or the like into an optical fiber.
2. Description of the Prior Art
In order to guide scattered light from a light source such as a light emitting diode, a semiconductor laser or the like into a core of an optical fiber through an optical coupler, axial and nonaxial aberrations of a lens constituting the optical coupler must be satisfactorily corrected so as to guide not only axial incident light but also nonaxial incident light due to assembly error or the like. In particular, in the case of a single mode optical fiber, since a core diameter is very small, the lens must satisfy a strict aberration performance requirement. In addition, the optical coupler must be compact light in weight and of low cost. Conventionally, an optical coupler using a graded index lens having parallel surfaces as two end faces is known as an optical coupler of this type.
However, in the case of a graded index lens having parallel surfaces, it is difficult to control the refractive index distribution so as to decrease the axial aberration. Even if the axial aberration becomes small, since the nonaxial aberration, more specifically, a coma is large, light outputted from the light source cannot be focused to a single point when the light source is shifted from the optical axis.
An example of a lens having one spherical surface is disclosed in Japanese Patent Disclosure No. 54-21751. In this lens, the spherical surface is used for correcting only a spherical aberration of the graded index lens. Therefore, although the spherical aberration can be corrected, the coma cannot be corrected.
An optical system which satisfies the above requirements can be constituted by 3 to 4 spherical lenses having a constant refractive index. However, such a combination of spherical lenses results in a large optical system, thus preventing a compact, lightweight system. Since the system has many lenses, assembly of the optical system and grinding of lens surfaces require a large number of steps, resulting in increased cost.